Federal agency repositories may contain peer-reviewed articles, data, or gray literature (technical reports, presentations, and more).   With the passage of the OSTP Memo, "Increasing Access to the Results of Federally Funded Scientific Research,"  agencies are creating policies that require recipients of federal grants to deposit their articles and data to these repositories within a certain timeframe.  See the Public Access to Federally Funded Research Guide for links to agency policies and more details.